Spacecraft require equipment that calculates the orbital position, velocity, time, and attitude. Traditionally, ground-based tracking systems are used to determine the spacecraft's position. The advent of the NAVSTAR satellite system, otherwise known as the Global Positioning System ("GPS"), increased the desirability of using onboard processing systems for spacecraft position determination.
The GPS satellite system is a collection of satellites that can be used for missile, satellite, aircraft, and terrestrial navigation. Each GPS satellite broadcasts its own ephemeris and time, thereby allowing a GPS receiver to determine its position. Typically, a GPS receiver calculates its position from the simultaneous observation of any four GPS satellites in order to calculate its position. A civil grade GPS receiver can accurately determine its position within 20 meters, determine its velocity within 0.6 meters/second, and the current time with a 10-nanosecond accuracy. Usually, twenty-four valid GPS satellites are in the GPS almanac at any given time.
A GPS receiver is typically an autonomous instrument that transforms signals from GPS satellites into point solutions for spacecraft. Current GPS receivers have a radio frequency section for receiving and converting signals received from a spacecraft's antennas. The digitized signals are then forwarded to one or more correlators, controlled by the receiver's own processor. The correlators look for matches between the incoming signal and the C/A code for different satellites. When a satellite lock occurs, or the incoming signal matches an internally generated pseudo random noise (PRN) code, the receiver's processor is notified. The processor contains executable code to generate a pseudo-range or a line-of-sight distance to the satellite. The processor will also contain the executable code of an orbit propagator. The pseudo-range is an input to the navigation solution that calculates point solutions for tracking the orbit of the spacecraft.
Previous spaceborne GPS receivers have been limited in that they must operate in a near nadir orientation. In previous designs, in order for a GPS receiver to determine its position, it must acquire the signals of at least three GPS satellites. Previous spaceborne GPS receivers assume that they are positioned in nadir orientation, and make certain assumptions as to particular GPS satellite availability. It is these assumptions that precipitate an increase in GPS satellite search and acquisition time, as the particular GPS satellite that a receiver is attempting to acquire may be outside the antenna envelope and/or blocked by the spacecraft. If a GPS receiver of previous design acquires one GPS satellite, it still needs to acquire two more satellites for a solution.
For example, if the antenna of a current GPS receiver is pointing parallel to the horizon, then additional problems are made clearer. Algorithms in current GPS receivers assume a nadir orientation, and thus the satellites available for acquisition are quite different than if the antenna is pointing nadir. The GPS receiver attempts to acquire GPS satellites that will not be in the location predicted by the receiver's algorithms or receivable by the antennas.
To acquire a GPS satellite, three dimensions of search space must be examined: GPS satellite number, code phase, and carrier frequency. Unless all three values match the incoming signal, no correlation will occur.
Referring to FIG. 1, there are 1023 shifts in the GPS C/A code phase. One GPS C/A code phase represents 300 meters or one second of error. A significant portion of the correlation time must be spent on the correct code shift phase in order to have a successful correlation. A factor of four is an accepted value to shift through the code phase. Thus, the correlator must "check" approximately four thousand different code shifts.
An additional problem faced by spaceborne GPS receivers is the increased range of Doppler shift that occurs in a spacecraft. To obtain satisfactory signal correlation, the GPS receiver must correlate the incoming GPS signal to within approximately 500 hertz before a traditional phase-lock loop or frequency-lock loop can lock onto the GPS signal. A GPS receiver on the ground encounters a Doppler shift on the order of .+-.5 kilohertz, which is easily handled. The GPS receiver will only have to examine approximately twenty frequency rows (i.e., frequency bands of 500 hertz that cover a total frequency range of 10 kilohertz) in order to acquire the desired GPS signal. One 500-hertz frequency row represents a 100-meter/second velocity error along the line-of-sight between a GPS receiver and a GPS satellite. However, in a low circular Earth orbit, the Doppler shift encountered is on the order of .+-.45 kilohertz. Referring to FIG. 1, two hundred frequency rows (i.e., frequency bands of 500 hertz that cover a total frequency range of 100 kilohertz) are required to acquire an incoming GPS signal.
Additionally, since each "dimension" of search space is independent of the other dimension, the number of bins to check for a low Earth orbit GPS receiver is approximately twenty million (25 satellites*4000 code shifts*200 frequency rows). A bin is a frequency row that is searched for a particular code shift. Correlation time also has a minimum value of approximately one millisecond to see any statistically meaningful value emerge. Thus, to search the full space with a single correlator channel, it would require over five and one-half hours. However, since the period of a low Earth orbit is approximately ninety minutes, this presents a significant problem.
In addition, due to spacecraft motion, GPS satellites are in view for much shorter time periods than when viewed from a ground reference. The appearance and disappearance of GPS satellites from view causes their output signals to slew through the entire range of the 45-kilohertz Doppler shift. As described above, when a GPS signal is acquired, the aggregate correlation time must exceed one millisecond in order to achieve a statistically meaningful value. Current GPS receivers are unable to accommodate such large Doppler shifts in their processing and their processing capacity prevents rapid shifting between acquired satellites.